In bottom portion of inland and coastal water where decomposition of organic matter containing sulfuric takes place (usually anaerobic decomposition) hydrogen sulfide (H2S) concentrations can be high. This can also be high in sewage contaminated waters. The gas can be detected by its characteristic odour and is often associated with black mud and sand and anoxic conditions. The gas is toxic to fish. Aquaculturists must be vary especially if there are inflows of sewage. Mangrove swamps often have H2S affected areas which should be checked properly for pond constuction. H2S combined with lack of oxygen in water can cause severe mortality and also turn the waters acidic. Site selection especially in the coastal area should be carefully done avoiding or correcting H2S affected areas. (see also “Mineral acidity” and “acid sulfate soils”).
H2S is produced in ponds by chemical reduction of organic matter accumulating in pond bottom, bottom soil often turning black and emanating a rotten smell.
Shigueno (1975) observed that shrimp (P. japonicus) larvae (often at bottom of ponds) lost equilibrium when exposed to a level of 0.1 to 2.0 ppm H2S in water. Instant death took place at 4 ppm.
In a pond where die-off of shrimps occurred, pond water showed H2S of 0.037 to 0.093 ppm, but the interstitial water, 2 cm below pond bottom had an H2S level of 10 ppm. Application of iron oxide at 1 kg per M2 prevented formation of H2S and shrimps in treated ponds grew better than those in untreated (average weight again and mortality in 68 days) in treated ponds: 204% and 4.4% respectively - same for untreated controls: 150% and 20.8%. See Table IV for relation of pH/temperature and hydrogen sulphide.
Table IV: Percentage un-ionized hydrogen sulfide in aqueous solution at different pH*
VALUES AND TEMPERATURE
* From Boyd (1979).